1. Field of the Invention
This invention relates generally to electronic devices and environment-dependent methods of maintaining the devices and indicating operational status of the devices. In particular, this invention relates to temperature-dependent methods of maintaining battery-operated external defibrillators and indicating their operational status.
2. Description of the Prior Art
Electronic devices that are infrequently used may be designed to perform automatic self-tests on a preset schedule, in response to an event or condition, or otherwise and to indicate the results of those self-tests to a potential user. An example can be found in certain external defibrillators that automatically self-test battery capacity and other defibrillator functions and components and indicate the results of those self-tests (i.e., the device's operational status) through visual displays and/or audible tones.
U.S. patent application Ser. No. 08/240,272 describes a battery-operated automatic external defibrillator (AED) designed for infrequent use. The device described in that patent application performs a variety of daily, weekly and monthly self-tests while in stand-by mode (i.e., when not powered-on to treat a patient, to review past treatment events, etc.) and indicates the operational status of the device using an "OK" or "Not OK" fail-safe visual display and using an audible tone generator. One of the device parameters monitored during the self-tests is remaining battery capacity.
The '272 application also suggests performing a group of self-tests automatically in response to exposure of the defibrillator to temperature extremes, although the exact nature of the environmentally-triggered self-tests is not disclosed. The disclosure of the '272 application is incorporated herein by reference.
There are many other types of battery-operated devices as well. Some battery-operated devices automatically track remaining battery capacity and indicate device status or make some other change based, at least in part, on remaining battery capacity. See, e.g., U.S. Pat. No. 3,895,284; U.S. Pat. No. 4,207,514; U.S. Pat. No. 4,525,055; U.S. Pat. No. 4,693,119; U.S. Pat. No. 4,725,784; U.S. Pat. No. 4,931,737; U.S. Pat. No. 5,065,084; U.S. Pat. No. 5,130,659; U.S. Pat. No. 5,162,741; and U.S. Pat. No. 5,483,165. The disclosures of these patents are incorporated herein by reference.
For example, Bates U.S. Pat. No. 5,162,741 describes a battery monitor that continuously samples the battery load current and temperature. The device continuously displays remaining battery capacity based on a temperature-compensated measurement of the amount of current drawn from the battery.
Hishiki U.S. Pat. No. 4,931,737 describes a battery capacity measurement circuit using a capacitor having thermal and age-variation characteristics to compensate for the thermal and age-variation characteristics of the battery. Specifically, the capacitor's capacitance, like the battery capacity, is maximum at room temperature and decreases with either an increase or decrease in ambient temperature. The capacitor is used to generate a pulse signal whose frequency varies with temperature as the capacitance (and therefore the battery capacity) changes, with the frequency being lowest at room temperature and increasing with an increase or decrease in ambient temperature. The pulse signals are counted by a counter to compute battery discharge and, thereby, remaining battery capacity.
Landau et al. U.S. Pat. No. 5,454,710 describes a battery monitoring and display system which adjusts actual measurements of remaining battery capacity by fixed percentages when the ambient temperature is in certain defined ranges.